Contacts for highly scaled transistors

ABSTRACT

A semiconductor device and methods of forming the same are disclosed. The semiconductor device includes a substrate, first and second source/drain (S/D) regions, a channel between the first and second S/D regions, a gate engaging the channel, and a contact feature connecting to the first S/D region. The contact feature includes first and second contact layers. The first contact layer has a conformal cross-sectional profile and is in contact with the first S/D region on at least two sides thereof. In embodiments, the first contact layer is in direct contact with three or four sides of the first S/D region so as to increase the contact area. The first contact layer includes one of a semiconductor-metal alloy, an III-V semiconductor, and germanium.

This claims the benefit of U.S. Prov. No. 62/081,348 entitled “Contactsfor Highly Scaled Transistors,” filed Nov. 18, 2014, herein incorporatedby reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

For example, multi-gate field effect transistors (FET) have beendeveloped for their high drive currents, small footprints, and excellentcontrol of short-channel effects. Examples of multi-gate FET include thedouble-gate FET, the triple-gate FET, the omega-gate FET, and thegate-all-around (or surround-gate) FET including both the horizontalgate-all-around (HGAA) FET and the vertical gate-all-around (VGAA) FET.The multi-gate FETs are expected to scale the semiconductor processtechnology beyond the limitations of the conventional bulkmetal-oxide-semiconductor FET (MOSFET) technology. However, as thetransistor device structure scales down and becomes three dimensional,the transistor contact resistance exhibits increased impact on thedevice performance. With conventional contact formation scheme,transistor contact resistance in highly scaled multi-gate FETs may limitthe devices' intrinsic performance well over 50%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 shows a flow chart of a method of fabricating a semiconductordevice, according to various aspects of the present disclosure.

FIGS. 2A, 2B, 2C, 3A, 3B, 4A, 4B, 5A, and 5B are perspective andcross-sectional views of forming a semiconductor device according to themethod of FIG. 1, in accordance with some embodiments.

FIGS. 6A, 6B, 7A, 7B, 8A, 8B, 9A, and 9B are cross-sectional views offorming a semiconductor device according to the method of FIG. 1, inaccordance with some embodiments.

FIGS. 10A and 10B are cross-sectional views of a semiconductor deviceconstructed in accordance with some embodiments of the method of FIG. 1.

FIGS. 10C, 10D, 10E, 10F, 10G, 10H, 10I, 10J, 10K, 10L, 10M, 10N, 10O,and 10P are cross-sectional views of semiconductor devices constructedin accordance with some embodiments of the method of FIG. 1.

FIGS. 11A and 11B are cross-sectional views of another semiconductordevice constructed in accordance with some embodiments of the method ofFIG. 1.

FIG. 12 shows a flow chart of a method of fabricating a semiconductordevice, according to various aspects of the present disclosure.

FIGS. 13A, 13B, 14A, 14B, 15, 16, 17, 18, and 19 are perspective andcross-sectional views of forming a semiconductor device according to themethod of FIG. 2, in accordance with some embodiments.

FIG. 20 is a cross-sectional view of another semiconductor device formedwith an embodiment of the method of FIG. 2.

FIGS. 21, 22, 23, 24, and 25 are cross-sectional views of forming thesemiconductor device of FIG. 20, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is generally related to semiconductor devices,and more particularly to semiconductor devices having multi-gatetransistors such as horizontal multi-gate transistors and verticalmulti-gate transistors. Examples of horizontal multi-gate transistorsinclude the double-gate FET, the triple-gate FET, the omega-gate FET,and the horizontal gate-all-around (HGAA) FET. Examples of verticalmulti-gate transistors include the vertical gate-all-around (VGAA) FETand tunneling FET (TFET). Furthermore, the HGAA FET and VGAA FET mayinclude one or more of the nanowire channel, the bar-shaped channel, orother suitable channel structures. An objective of the presentdisclosure is to provide novel source/drain (S/D) contacts for themulti-gate transistors, wherein the novel S/D contacts have reducedcontact resistance compared to the conventional S/D contacts.

In the following discussion, various embodiments of the presentdisclosure are described in the context of fabricating devices 100, 200,300, 400, 500, 600, and 700. These devices are non-limiting examplesthat can be manufactured with some embodiments of the presentdisclosure. Furthermore, each of the devices 100, 200, 300, 400, 500,600, and 700 may be an intermediate device fabricated during processingof an integrated circuit (IC), or a portion thereof, that may comprisestatic random access memory (SRAM) and/or other logic circuits, passivecomponents such as resistors, capacitors, and inductors, and activecomponents such as p-type FETs, n-type FETs, metal-oxide semiconductorfield effect transistors (MOSFET), complementary metal-oxidesemiconductor (CMOS) transistors, bipolar transistors, high voltagetransistors, high frequency transistors, other memory cells, andcombinations thereof.

First Embodiment

The first embodiment of the present disclosure is now described withreference to FIGS. 1-5B in fabricating the device 100. FIG. 1 shows aflow chart of a method 10 of forming a semiconductor device, such as asemiconductor device having a multi-gate structure, according to variousaspects of the present disclosure. The method 10 is merely an example,and is not intended to limit the present disclosure beyond what isexplicitly recited in the claims. Additional operations can be providedbefore, during, and after the method 10, and some operations describedcan be replaced, eliminated, or moved around for additional embodimentsof the method.

At operation 12, the method 10 (FIG. 1) receives the device 100 as shownin FIGS. 2A, 2B and 2C, wherein FIG. 2A is a perspective schematic viewof the device 100, FIG. 2B is a cross-sectional view of the device 100along the “A-A” line of FIG. 2A, and FIG. 2C is a cross-sectional viewof the device 100 along the “B-B” line of FIG. 2A. Referring to FIGS.2A, 2B, and 2C collectively, the device 100 includes a substrate 102, afin 104, an isolation structure 106, a gate 108, and a dielectric layer110. The fin 104 projects upwardly (along the “z” direction) from thesubstrate 102. The isolation structure 106 is disposed over thesubstrate and adjacent to a bottom portion of the fin 104. It isolatesthe fin 104 from other active regions (not shown) of the device 100. Thegate 108 is formed over the isolation structure 106 and engages the fin104 on three sides thereof. Therefore, the device 100 as shown is atriple-gate device. Other types of gate structures, such as double-gate(e.g., the gate 108 engages two side surfaces of the fin 104),omega-gate (e.g., the gate 108 fully engages a top surface and two sidesurfaces of the fin 104 and partially engages a bottom surface of thefin 104), and gate-all-around (e.g., the gate 108 fully engages top,bottom, and two side surfaces of the fin 104) are within the scope ofthe present disclosure. The dielectric layer 110 is disposed over thefin 104, the isolation structure 106, and the gate 108. The variouselements of the device 100 will be further described in the followingsections.

The substrate 102 is a silicon substrate in the present embodiment.Alternatively, the substrate 102 may comprise another elementarysemiconductor, such as germanium; a compound semiconductor includingsilicon carbide, gallium arsenic, gallium phosphide, indium phosphide,indium arsenide, and/or indium antimonide; an alloy semiconductorincluding SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; orcombinations thereof.

The fin 104 is suitable for forming an n-type FET or a p-type FET. Thefin 104 may be fabricated using suitable processes includingphotolithography and etch processes. The photolithography process mayinclude forming a photoresist layer (resist) overlying the substrate102, exposing the resist to a pattern, performing post-exposure bakeprocesses, and developing the resist to form a masking element includingthe resist. The masking element is then used for etching recesses intothe substrate 102, leaving the fin 104 on the substrate 102. The etchingprocess can include dry etching, wet etching, reactive ion etching(RIE), and/or other suitable processes. Alternatively, the fin 104 maybe formed using mandrel-spacer double patterning lithography. Numerousother embodiments of methods to form the fin 104 may be suitable.

The isolation structure 106 may be formed of silicon oxide, siliconnitride, silicon oxynitride, fluoride-doped silicate glass (FSG), alow-k dielectric material, and/or other suitable insulating material.The isolation structure 106 may be shallow trench isolation (STI)features. In an embodiment, the isolation structures 106 is formed byetching trenches in the substrate 102, e.g., as part of the fin 104formation process. The trenches may then be filled with isolatingmaterial, followed by a chemical mechanical planarization (CMP) process.Other isolation structure such as field oxide, LOCal Oxidation ofSilicon (LOCOS), and/or other suitable structures are possible. Theisolation structure 106 may include a multi-layer structure, forexample, having one or more thermal oxide liner layers.

The fin 104 and the gate 108 are further illustrated with reference toFIG. 2B. Referring to FIG. 2B, the fin 104 includes two source/drain(S/D) regions (or features) 104 a and a channel region 104 b between thetwo S/D regions 104 a. The S/D regions 104 a and the channel region 104b are arranged in a horizontal manner (along the “y” direction) over theisolation structure 106. Therefore, the device 100 is a horizontalmulti-gate device. The gate 108 includes a gate stack 108 a and a gatespacer 108 b on sidewalls of the gate stack 108 a. The gate stack 108 aengages the fin 104 at the channel region 104 b. In various embodiments,the gate stack 108 a includes a multi-layer structure. In one example,the gate stack 108 a includes an interfacial layer and a poly-siliconlayer. In another example, the gate stack 108 a includes an interfaciallayer, a high-k dielectric layer, a barrier layer, a work function metallayer, and a metal fill layer. Various other embodiments of the gatestack 108 a are possible. The gate stack 108 a may be formed usingeither a “gate-first” or a “gate-last” method. In embodiments, the gatespacer 108 b includes a dielectric material, such as silicon nitride orsilicon oxynitride and is formed by one or more deposition and etchingprocesses.

The dielectric layer 110, also referred to as an inter-layer dielectric(ILD) layer, is disposed over the various structures discussed above. Inembodiments, the device 100 further includes a contact etch stop (CES)layer underneath the ILD layer 110. The ILD layer 110 may includematerials such as tetraethylorthosilicate (TEOS) oxide, un-dopedsilicate glass, or doped silicon oxide such as borophosphosilicate glass(BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), borondoped silicon glass (BSG), and/or other suitable dielectric materials.The ILD layer 110 may be deposited by a plasma-enhanced chemical vapordeposition (PECVD) process or other suitable deposition technique. In anembodiment, the ILD layer 110 is formed by a flowable CVD (FCVD)process. The FCVD process includes depositing a flowable material (suchas a liquid compound) on the substrate 102 to fill trenches andconverting the flowable material to a solid material by a suitabletechnique, such as annealing in one example. After various depositionprocesses, a chemical mechanical planarization (CMP) process isperformed to planarize a top surface of the ILD layer 110.

At operation 14, the method 10 (FIG. 1) etches the ILD layer 110 to forman opening (or a contact hole) 112. Referring to FIGS. 3A and 3B, FIG.3A is a cross-sectional view of the device 100 along the “A-A” line ofthe FIG. 2A after the operation 14, and FIG. 3B is a cross-sectionalview of the device 100 along the “B-B” line of the FIG. 2A after theoperation 14. The opening 112 has a bottom surface 112′ that is below atop surface 104 a′ of the fin 104 a. The portion of the fin 104 exposedin the opening 112 has a height “R” which is also the vertical distancebetween the bottom surface 112′ and the top surface 104 a′ along the zdirection. The portion of the fin 104 above the isolation structure 106has a height “F.” In embodiments, R is greater than half of F. In someembodiments, R ranges from about 5 nanometer (nm) to about 60 nm. In anexample, the opening 112 may be etched into the isolation structure 106.The opening 112 is deeper than conventional contact holes whichtypically stop at the top surface 104 a′. One benefit of having a deepopening 112 is that an S/D contact formed therein will have largercontact areas with the S/D region 104 a.

In various embodiments, the opening 112 has a top width T and a bottomwidth B along the x direction, and a height H along the z direction. Thebottom width B is greater than the width w_(f) of the fin 104 a alongthe x direction. The top width T is greater than the bottom width B.Accordingly, the sidewalls of the opening 112 are slanted. Thedimensions T, B, and H should be designed such that all surfaces of theopening 112 are easily accessible when a conductive material isdeposited into the opening 112 to form a contact, as will be shownlater. For the same consideration, the distances, b₁ and b₂, from thesidewalls of the opening 112 to the sidewalls of the fin 104 a aredesigned such that the bottom and sidewalls of the opening 112 as wellas the sidewalls of the fin 104 a are easily accessible during thedeposition of the conductive material. In various embodiments, T rangesfrom about 12 to about 40 nm, B ranges from about 8 to about 30 nm, andH ranges from about 50 to about 150 nm. In various embodiments, b1 andb2 each ranges from about half (½) of w_(f) to about one and half (1½)of w_(f). In addition, although FIG. 3B shows the opening 112 to beabout symmetrical about the fin 104 a in the z-x plane, this is merelyexemplary in nature and does not limit the present disclosure. Forexample, in embodiments, b₁ and b₂ may be different.

The etching processes may include a suitable wet etch, dry (plasma)etch, and/or other processes. For example, a dry etching process may usechlorine-containing gases, fluorine-containing gases, other etchinggases, or a combination thereof. The wet etching solutions may includeNH₄OH, HF (hydrofluoric acid) or diluted HF, deionized water, TMAH(tetramethylammonium hydroxide), other suitable wet etching solutions,or combinations thereof.

In an embodiment, the device 100 includes a contact etch stop (CES)layer underneath the ILD layer 110 but over the S/D regions 104 and overthe gate 108. For example, the CES layer may be made of a materialsimilar to that for the isolation structure 106, such as silicon oxideor silicon nitride. During the operation 14, the CES layer protects thefin 104 from over-etching. If a contact hole to the gate 108 is etchedat the same time, the CES layer further protects the gate 108 fromover-etching. To further this embodiment, the operation 14 furtherincludes an etching process tuned to remove the CES layer within theopening 112, thereby exposing the S/D regions 104 a for contactformation.

In yet another embodiment, the device 100 includes a contact etch stop(CES) layer over the S/D regions 104 and over the gate 108. Prior toforming the ILD layer 110, the method 10 partially removes the CES layerso that the S/D regions 104 a are exposed to provide top and sidewallsurfaces for subsequent S/D contact formation. To further thisembodiment, once the operation 14 removes the ILD layer 110 to form theopening 112, the fin surfaces for S/D contact formation are exposed.

At operation 16, the method 10 (FIG. 1) forms a first contact layer 114in the opening 112. Referring to FIGS. 4A and 4B, FIG. 4A is across-sectional view of the device 100 along the “A-A” line of the FIG.2A after the operation 16, and FIG. 4B is a cross-sectional view of thedevice 100 along the “B-B” line of the FIG. 2A after the operation 16.The first contact layer 114 is formed over the surfaces of the opening112. In particular, it is formed over the top surface and sidewalls ofthe S/D region 104 a. The first contact layer 114 has a conformalprofile, i.e. it has a near uniform thickness over the surfaces of theopening 112. In an embodiment, the first contact layer 114 has athickness ranging from about 2 nm to about 10 nm. In an embodiment, thefirst contact layer 114 includes a semiconductor-metal alloy. Forexample, the semiconductor-metal alloy may include a metal material suchas titanium, cobalt, nickel, nickel cobalt, other metals, or acombination thereof. To further this embodiment, the metal material isdeposited using a chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), or other suitabledeposition techniques. Then, an annealing process is performed therebyforming a semiconductor-metal alloy over the surfaces of the S/D regions104 a. In another embodiment, the first contact layer 114 includes oneor more III-V semiconductors that provide high carrier mobility and/orsuitable band structure for tuning energy barrier. For example, thefirst contact layer 114 may include InAs, InGaAs, InP, or other suitableIII-V semiconductors. In yet another embodiment, the first contact layer114 includes germanium (Ge). In various embodiments, the first contactlayer 114 may be deposited using CVD, PVD, ALD, or other suitablemethods. In various embodiments, the material of the first contact layer114 offers low or negligible energy barrier for charge carriers flowinginto and out of the transistor channel. The first contact materialcoupled with increased contact area reduces the contact resistance tothe S/D regions 104 a.

At operation 18, the method 10 (FIG. 1) forms a second contact layer 116in the opening 112 over the first contact layer 114. Referring to FIGS.5A and 5B, FIG. 5A is a cross-sectional view of the device 100 along the“A-A” line of the FIG. 2A after the operation 18, and FIG. 5B is across-sectional view of the device 100 along the “B-B” line of the FIG.2A after the operation 18. The second contact layer 116 fills theremaining space of the opening 112. The second contact layer 116 mayinclude one or more layers of metallic materials, such as metallicnitrides, metallic or conductive oxides, elemental metals, orcombinations thereof. For example, the second contact layer 116 may usetungsten (W), copper (Cu), cobalt (Co), and/or other suitable materials.In various embodiments, the second contact layer 116 may be formed byCVD, PVD, plating, and/or other suitable processes. As shown in FIGS. 5Aand 5B, an S/D contact 118 is formed in each of the openings 112,conductively connecting to the respective S/D regions 104 a. The S/Dcontact 118 includes the first contact layer 114 and the second contactlayer 116. Various dimensions of the S/D contact 118 are labeled in FIG.5B, including a top width “T,” a bottom width “B,” and a height “H.” Therelationship among T, B, H, and the width w_(f) of the fin 104 a hasbeen discussed with reference to FIG. 3B. In various embodiments, Tranges from about 12 to about 40 nm, B ranges from about 8 to about 30nm, and H ranges from about 50 to about 150 nm.

At operation 20, the method 10 (FIG. 1) performs further steps tocomplete the fabrication of the device 100. For example, the operation20 may form a gate contact electrically connecting the gate stack 108 a,and may form metal interconnects connecting the multi-gate FET to otherportions of the device 100 to form a complete IC.

Second Embodiment

The second embodiment of the present disclosure is now described withreference to FIGS. 1 and 6A-9B, wherein the device 200 is fabricatedaccording to some embodiments of the method 10. FIGS. 6A-9B illustratecross-sectional views of the device 200 in the process of fabrication.Discussions applicable to both the devices 100 and 200 are abbreviatedor omitted below for the sake of simplicity.

At the operation 12, the method 10 (FIG. 1) receives the device 200,which is similar to the device 100 (FIGS. 2A-2C) in many respects. Forthe purpose of simplicity, same reference numerals are used to labelsimilar elements of the two devices. For example, as shown in FIGS. 6Aand 6B, the device 200 also includes a substrate 102, a fin 104, anisolation structure 106, a gate 108, and an ILD layer 110. The gate 108also includes a gate stack 108 a and a gate spacer 108 b. The gate stack108 a engages a channel region 104 b of the fin 104. One differencebetween the devices 100 and 200 lies in the structure of the S/D regionsof the two devices. As shown in FIG. 6B, the device 200 hasdiamond-shaped S/D regions 204 a. In an embodiment, the S/D regions 204a are formed by etching a portion of the fin 104 of the device 200 toform recesses therein and epitaxially growing one or more semiconductorfeatures from the recesses. For example, the etching process may use adry etching, a wet etching, or other suitable etching methods. Acleaning process may be performed that cleans the recesses with ahydrofluoric acid (HF) solution or other suitable solution.Subsequently, one or more epitaxial growth processes are performed togrow semiconductor (e.g., silicon) features in the recesses. Theepitaxial growth process may in-situ dope the grown semiconductor with ap-type dopant for forming a p-type FET or an n-type dopant for formingan n-type FET. As further illustrated in FIG. 6B, the S/D regions 204 aeach have two upwardly facing surfaces (or sides) 204 a′ and twodownwardly facing surfaces (or sides 204 a″).

At the operation 14, the method 10 (FIG. 1) etches the ILD layer 110 ofthe device 200 to form an opening 112 therein. Referring to FIGS. 7A and7B, the opening 112 has a bottom surface 112′ that is below the surfaces204 a′. The portion of the fin 104/204 a exposed in the opening 112 hasa height “R.” The portion of the fin 104/204 a above the isolationstructure 106 has a height “F.” In embodiments, R is greater than halfof F. In embodiments, the opening 112 fully exposes the surface 204 a′and may partially or fully expose the surfaces 204 a″. In someembodiments, R ranges from about 5 nanometer (nm) to about 60 nm. Theopening 112 is deeper than conventional contact holes which typicallystop at the surfaces 204 a′. One benefit of having deeper openings 112is that S/D contacts formed therein will have larger contact areas withthe S/D regions 204 a. Other respects of this operation are similar tothose discussed with reference to FIGS. 3A and 3B.

At the operation 16, the method 10 (FIG. 1) forms a first contact layer114 in the opening 112. Referring to FIGS. 8A and 8B, the first contactlayer 114 is formed over the surfaces of the opening 112. In particular,it is formed over the surfaces 204 a′ and 204 a″ (FIG. 7B) of the S/Dregion 204 a. The first contact layer 114 has a conformal profile. In anembodiment, the first contact layer 114 has a thickness ranging fromabout 2 nm to about 10 nm. The material and formation of the firstcontact layer 114 are similar to those discussed with reference to FIGS.4A and 4B. In various embodiments, the material of the first contactlayer 114 offers low or negligible energy barrier for charge carriersflowing into and out of the transistor channel. The first contactmaterial coupled with increased contact area to the S/D regions 204 areduces the contact resistance thereof.

At the operation 18, the method 10 (FIG. 1) forms a second contact layer116 in the opening 112 over the first contact layer 114. Referring toFIGS. 9A and 9B, an S/D contact 118 is formed in each of the openings112, conductively connecting to the respective S/D regions 204 a. TheS/D contact 118 includes the first contact layer 114 and the secondcontact layer 116. Other respects of the contact 118, such asdimensions, are similar to those discussed with reference to FIGS. 5Aand 5B.

Third Embodiment

The third embodiment of the present disclosure is now described withreference to FIGS. 10A and 10B, wherein the device 300 has beenfabricated according to some embodiments of the method 10. Discussionsapplicable to both the devices 100 and 300 are abbreviated or omittedbelow for the sake of simplicity.

Referring to FIGS. 10A and 10B, the device 300 includes two horizontal(in the “x-y” plane) rod-shaped channels 304 b. In embodiments, thenumber of channels and the shape of the channels in the device 300 mayvary. For example, the channels 304 b may be bar-shaped or have othersuitable shapes, and there may be one or more channels. The device 300includes a gate 108 that wraps around the channels 304 b. Hence, thedevice 300 is a horizontal gate-all-around (HGAA) device. Other respectsof the device 300 are the same as or similar to those of the device 200.For example, the device 300 also includes diamond-shaped S/D regions 304a formed over the substrate 102 and the fin 104. The process of formingS/D contacts for the device 300 is the same as what have been discussedwith respect to the devices 100 and 200. An exemplary process of formingthe device 300 prior to the S/D contact formation can be found in U.S.Pat. No. 8,815,691 entitled “Method of Fabricating a Gate All AroundDevice,” the contents of which are hereby incorporated by reference intheir entirety.

Examples of the First, Second, and Third Embodiments

FIGS. 10C-10P show S/D regions of various devices (devices 320, 322,324, 326, 328, 330, 332, 334, 336, 338, 340, 342, 344, and 346respectively) constructed according to aspects of the presentdisclosure. Each of the devices 320-346 may have a channel region and agate stack constructed similar to the devices 100, 200, and 300. Forexample, each of the devices 320-346 may have a fin-like channel engagedby a gate stack on three sides of the channel, such as shown in FIG. 9A;or each of them may have a horizontal channel wrapped around by a gatestack, such as shown in FIG. 10A. Alternatively, each of the devices320-346 may have a channel region and a gate stack constructeddifferently from those of the devices 100, 200, and 300. The devices100, 200, 300, and 320-346 are non-limiting examples. Further examplesmay be constructed by combining, substituting, and/or reconfiguringvarious features of these devices. For the purposes of simplicity, onlythe S/D regions of the devices 320-346 are shown in the respectivefigures, which are described below.

Referring to FIG. 10C, the device 320 includes a substrate 102, two fins104, two S/D regions 314 a formed over the two fins 104, an isolationstructure 106, an ILD layer 110, a first contact layer 114, and a secondcontact layer 116. The fins 104 extend above a top surface of theisolation structure 106. The S/D regions 314 a each have a diamond shapeand are disposed over top surfaces of the fins 104. The first contactlayer 114 wraps around all surfaces (or sides) of the S/D regions 314 a.A gap between the S/D regions 314 a has a dimension (along the “y”direction) greater than twice of the thickness of the first contactlayer 114. Further, another gap between the S/D regions 314 and the ILDlayer 110 has a dimension (along the “y” direction) greater than twiceof the thickness of the first contact layer 114. The device 320 may beformed by an embodiment of the method 10 (FIG. 1). For example, a deviceprecursor 320 is received at the operation 12, which includes thesubstrate 102, the isolation structure 106, the fins 104, the S/Dregions 314 a, and the ILD layer 110. The S/D regions 314 a are buriedin the ILD layer 110. Subsequently, the ILD layer 110 is etched at theoperation 14 to expose all surfaces of the S/D regions 314 a. Next, thefirst contact layer 114 is formed at the operation 16. The first contactlayer 114 wraps around all surfaces of the S/D regions 134. Thereafter,the second contact layer 116 is formed over the first contact layer 114.Even though FIG. 10C illustrates the device 320 having two fins 104, invarious embodiments, the device 320 may include any number of fins 104,such as one fin, two fins, three fins, and so on. In one example, thedevice 320 may include one hundred fins 104.

Referring to FIG. 10D, the device 322 includes a substrate 102, two fins104, two S/D regions 314 a formed over the two fins 104, an isolationstructure 106, an ILD layer 110, a first contact layer 114, and a secondcontact layer 116. Top surfaces of the fins 104 and a top surface of theisolation structure 106 are substantially co-planar. The S/D regions 314a each have a diamond shape and are disposed over the top surfaces ofthe fins 104. The first contact layer 114 fully covers two upwardlyfacing surfaces of the S/D regions 314 a, but only partially covers twodownwardly facing surfaces of the S/D regions 314 a. A gap between theS/D regions 314 a has a dimension (along the “y” direction) less thantwice of the thickness of the first contact layer 114. As a result, therespective portions of the first contact layer 114 (on surfaces of thetwo S/D regions 314 a) merge in the gap. Further, another gap betweenthe S/D regions 314 and the ILD layer 110 has a dimension (along the “y”direction) less than twice of the thickness of the first contact layer114. As a result, the respective portions of the first contact layer 114(on sidewalls of the ILD layer 110 and on surfaces of the S/D regions314 a) merge in the gap. The device 322 may be formed by an embodimentof the method 10 (FIG. 1), as discussed above. Further, in variousembodiments, the device 322 may include any number of fins 104, such asone fin, two fins, three fins, and so on. In one example, the device 322may include one hundred fins 104.

Referring to FIG. 10E, the device 324 includes a substrate 102, two fins104, two S/D regions 314 a formed over the two fins 104, an isolationstructure 106, an ILD layer 110, a first contact layer 114, and a secondcontact layer 116. Top surfaces of the fins 104 and a top surface of theisolation structure 106 are substantially co-planar. The S/D regions 314a each have a diamond shape and are disposed over the top surfaces ofthe fins 104. Portions of the S/D regions 314 a merge. A space (or gap)316 is formed below the merged portion, surrounded by two downwardlyfacing surfaces 314 a′ of the S/D region 314 a and the top surface ofthe isolation structure 106. The first contact layer 114 fully coversupwardly facing surfaces of the S/D regions 314 a, but only partiallycovers downwardly facing surface 314 a″ of each of the S/D regions 314a. Further, a gap between the S/D regions 314 a and the ILD layer 110has a dimension (along the “y” direction) less than twice of thethickness of the first contact layer 114. As a result, the respectiveportions of the first contact layer 114 (on sidewalls of the ILD layer110 and on surfaces of the S/D regions 314 a) merge in the gap. Thedevice 324 may be formed by an embodiment of the method 10 (FIG. 1), asdiscussed above. Further, in various embodiments, the device 324 mayinclude any number of fins 104, such as one fin, two fins, three fins,and so on. In one example, the device 324 may include one hundred fins104.

Referring to FIG. 10F, the device 326 includes a substrate 102, two fins104, a S/D region 314 a formed over the two fins 104, an isolationstructure 106, an ILD layer 110, a first contact layer 114, and a secondcontact layer 116. Top surfaces of the fins 104 and a top surface of theisolation structure 106 are substantially co-planar. The S/D region 314a has a hexagonal shape in the “z-y” plane with a top surface, a bottomsurface, two upwardly facing surfaces, and two downwardly facingsurfaces. The top and bottom surfaces of the S/D region 314 a aresubstantially parallel to the “x-y” plane (see FIG. 2A). The bottomsurface of the S/D region 314 a is disposed over the top surfaces of thefins 104. The first contact layer 114 fully covers the top surface andthe two upwardly facing surfaces of the S/D region 314 a, but onlypartially covers the two downwardly facing surfaces of the S/D region314 a. Further, a gap between the S/D region 314 a and the ILD layer 110has a dimension (along the “y” direction) less than twice of thethickness of the first contact layer 114. As a result, the respectiveportions of the first contact layer 114 (on sidewalls of the ILD layer110 and on surfaces of the S/D region 314 a) merge in the gap. Thedevice 326 may be formed by an embodiment of the method 10 (FIG. 1), asdiscussed above. Further, in various embodiments, the device 326 mayinclude any number of fins 104, such as one fin, two fins, three fins,and so on. In one example, the device 326 may include one hundred fins104.

Referring to FIG. 10G, the device 328 includes a substrate 102, a fin104, an S/D region 314 a formed over the fin 104, an isolation structure106, an ILD layer 110, a first contact layer 114, and a second contactlayer 116. A top surface of the fin 104 and a top surface of theisolation structure 106 are substantially co-planar. The S/D region 314a has a hexagonal shape in the “z-y” plane with a top surface, a bottomsurface, two upwardly facing surfaces, and two downwardly facingsurfaces. The top and bottom surfaces of the S/D region 314 a aresubstantially parallel to the “x-y” plane (see FIG. 2A). The bottomsurface of the S/D region 314 a is disposed over the top surface of thefin 104. The first contact layer 114 fully covers the top surface andthe two upwardly facing surfaces of the S/D region 314 a, but onlypartially covers the two downwardly facing surfaces of the S/D region314 a. Further, a gap between the S/D region 314 a and the ILD layer 110has a dimension (along the “y” direction) less than twice of thethickness of the first contact layer 114. As a result, the respectiveportions of the first contact layer 114 (on sidewalls of the ILD layer110 and on surfaces of the S/D region 314 a) merge in the gap. Thedevice 328 may be formed by an embodiment of the method 10 (FIG. 1), asdiscussed above. Further, in various embodiments, the device 328 mayinclude any number of fins 104, such as one fin, two fins, three fins,and so on. In one example, the device 328 may include one hundred fins104.

Referring to FIG. 10H, the device 330 includes a substrate 102, two fins104, two S/D regions 314 a formed over the respective fins 104, anisolation structure 106, an ILD layer 110, a first contact layer 114,and a second contact layer 116. Top surfaces of the fins 104 and a topsurface of the isolation structure 106 are substantially co-planar. TheS/D regions 314 a each have a substantially hexagonal shape in the “z-y”plane with two upwardly facing surfaces, two side surfaces, and twodownwardly facing surfaces. The two upwardly facing surfaces are slantedfrom the “x-y” plane (see FIG. 2A) and meet to form a ridge. The twoside surfaces are substantially parallel to the “x-z” plane (see FIG.2A). The two downwardly facing surfaces are also slanted from the “x-y”plane. The first contact layer 114 fully covers the two upwardly facingsurfaces of each S/D region 314 a, but only partially covers the twoside surfaces of each S/D region 314 a. The device 330 may be formed byan embodiment of the method 10 (FIG. 1), as discussed above. Further, invarious embodiments, the device 330 may include any number of fins 104,such as one fin, two fins, three fins, and so on. In one example, thedevice 330 may include one hundred fins 104.

Referring to FIG. 10I, the device 332 is similar to the device 330 inmany respects. Some differences are noted below. In the device 332, thefirst contact layer 114 does not cover the two outer side surfaces 314a′ of the S/D region 314 a. The first contact layer 114 fully covers thetwo inner upwardly facing surfaces 314 a′″, but full or partially coversthe two inner side surfaces 314 a″ and the two outer upwardly facingsurfaces 314 a″″. The device 332 may be formed by an embodiment of themethod 10 (FIG. 1), as discussed above. For example, when etching theILD layer 110 at operation 14, the etching dimensions are controlledsuch that the surfaces 314 a′ are not exposed by the etching process.

Referring to FIG. 10J, the device 334 is similar to the device 332 inmany respects. Some differences are noted below. In the device 334, thefins 104 extend above a top surface of the isolation structure 106 andthe S/D regions 314 a are each disposed (e.g., by an epitaxial growthprocess) over the respective fins 104 without recessing the fins 104. Asa result, the S/D regions 314 a each wrap around the respective fins104. The device 334 may be formed by an embodiment of the method 10(FIG. 1), as discussed above.

Referring to FIG. 10K, the device 336 is similar to the device 320 (FIG.10C) in many respects. Some differences are noted below. In the device336, two outer downwardly facing surfaces 314 a′ of the S/D regions 314a are not covered by the first contact layer 114. The first contactlayer 114 fully covers two inner upwardly facing surfaces 314 a″ and twoinner downwardly facing surfaces 314 a′″, and partially or fully coverstwo outer upwardly facing surfaces 314 a″″. The device 336 may be formedby an embodiment of the method 10 (FIG. 1), as discussed above. Forexample, when etching the ILD layer 110 at operation 14, the etchingdimensions are controlled such that the surfaces 314 a′ are not exposedby the etching process.

Referring to FIG. 10L, the device 338 is similar to the device 322 (FIG.10D) in many respects. Some differences are noted below. In the device338, two outer downwardly facing surfaces 314 a′ of the S/D regions 314a are not covered by the first contact layer 114. The first contactlayer 114 fully covers two inner upwardly facing surfaces 314 a″, andfully or partially covers two inner downwardly facing surfaces 314 a′″and two outer upwardly facing surfaces 314 a″″.

Referring to FIG. 10M, the device 340 is similar to the device 324 (FIG.10E) in many respects. Some differences are noted below. In the device340, the first contact layer 114 fully covers two inner upwardly facingsurfaces of the S/D regions 314 a, and fully or partially covers twoouter upwardly facing surfaces of the S/D regions 314 a. Further, itdoes not cover the downwardly facing surfaces 314 a′ and 314 a″.

Referring to FIG. 10N, the device 342 is similar to the device 326 (FIG.10F) in many respects. Some differences are noted below. In the device342, the first contact layer fully covers the top surface of the S/Dregion 314 a, and fully or partially covers the two upwardly facingsurfaces of the S/D region 314 a. The first contact layer does not coverthe two downwardly facing surfaces of the S/D region 314 a.

Referring to FIG. 10O, the device 344 is similar to the device 328 (FIG.10G) in many respects. Some differences are noted below. In the device344, the first contact layer fully covers the top surface of the S/Dregion 314 a, and fully or partially covers the two upwardly facingsurfaces of the S/D region 314 a. The first contact layer does not coverthe two downwardly facing surfaces of the S/D region 314 a.

Referring to FIG. 10P, the device 346 is similar to the device 336 (FIG.10K) in many respects. Some differences are noted below. In the device346, the first contact layer 114 fully covers all surfaces of the S/Dregions 314 a. Further, the device 346 optionally includes a barriermetal layer 116 a between the second contact layer 116 and the ILD layer110 and between the second contact layer 116 and the first contact layer114. In an embodiment, the barrier metal layer 116 a includes a metalnitride (e.g., TaN) for preventing the metal elements of the secondcontact layer 116 from migrating to adjacent features. The barrier metallayer 116 a is conductive and has a conformal profile, similar to thefirst contact layer 114 of FIG. 10K. The device 346 may be formed by anembodiment of the method 10 (FIG. 1), as discussed above. For example, adevice precursor 346 is received at operation 12 (FIG. 1) that includesthe substrate 102, the fins 104, and the isolation structure 106. Thefins 104 extend above the top surface of the isolation structure 106.The device 346 further includes the S/D regions 314 a disposed over therespective fins 104. Next, the first contact layer 114 is formed (theoperation 16) to fully cover the surfaces of the S/D regions 314 a.Next, the ILD layer 110 is deposited over the device 346 and covers thefirst contact layer 114, the S/D regions 314 a, and the fins 104. Next,the ILD layer 110 is etched (the operation 14) to form an opening whichexposes portions of the first contact layer 114 except the portions onthe two outer downwardly facing surfaces 314 a′ of the S/D regions 314a. Next, the second contact layer 116 is formed in the opening (theoperation 18). In the present embodiment, the operation 18 includesforming the barrier metal layer 116 a (e.g., using CVD or PVDtechniques) before the formation of the second contact layer 116.

In various embodiments, each of the devices 322, 324, 326, 328, 330,332, 334, 336, 338, 340, 342, and 344 may be formed to have the firstcontact layer 114 fully wrapping the S/D regions 314 a before therespective ILD layer 110 is formed, such as discussed with reference toFIG. 10P.

Fourth Embodiment

The fourth embodiment of the present disclosure is now described withreference to FIGS. 11A and 11B, wherein the device 400 has beenfabricated according to some embodiments of the method 10. Discussionsapplicable to both the devices 100 and 400 are abbreviated or omittedbelow for the sake of simplicity.

Referring to FIGS. 11A and 11B, the device 400 includes two horizontal(in the “x-y” plane) rod-shaped active regions 404. Source and drainregions 404 a and channel 404 b are formed in the active regions 404 andhave the same rod shape. In embodiments, the number and shapes of theactive regions 404 may vary. For example, the active regions 404 mayhave a bar shape or other suitable shapes, and there may be one or moreof such active regions in the device 400. Similar to the device 300, thedevice 400 is also a HGAA device as its gate 108 wraps around thechannels 404 b. One difference between the devices 300 and 400 lies inthe configuration of their S/D regions. The S/D regions 404 a areisolated from the substrate 102 and the fin 104 at least within thecontact holes. Therefore, the first contact layer 114 wraps around eachof the S/D regions 404 a, providing the maximum contact area. As shownin FIGS. 11A and 11B, a portion 116A of the second contact layer 116fills the space between the S/D regions 404 a after the first contactlayer 114 have been formed around thereof. In another embodiment where avertical distance between the two S/D regions 404 a along the zdirection is not greater than two times of the thickness of the firstcontact layer 114, the first contact layer 114 around each of the S/Dregions 404 a physically contact each other. The process of forming theS/D contacts for the device 400 is the same as what have been discussedwith respect to the devices 100. An exemplary process of forming thedevice 400 prior to the S/D contact formation can be found in U.S. Pat.No. 8,815,691 entitled “Method of Fabricating a Gate All Around Device,”the contents of which are hereby incorporated by reference in theirentirety.

Fifth Embodiment

The fifth embodiment of the present disclosure is now described withreference to FIGS. 12-18. FIG. 12 shows a flow chart of a method 50 offorming a semiconductor device, particularly a semiconductor devicehaving a vertical multi-gate structure, according to various aspects ofthe present disclosure. The method 50 is merely an example, and is notintended to limit the present disclosure beyond what is explicitlyrecited in the claims. Additional operations can be provided before,during, and after each of the method 50, and some operations describedcan be replaced, eliminated, or moved around for additional embodimentsof the method.

At operation 52, the method 50 (FIG. 12) receives a vertical multi-gatedevice prior to the S/D contact formation. An exemplary verticalmulti-gate device, the device 500, is shown in FIGS. 13A and 13B. FIG.13A is a schematic perspective view of the device 500 and FIG. 13B is atop view of the device 500 (with the ILD layer 110 removed). The device500 includes a substrate 102, a first S/D region (or feature) 104 a as amesa on the substrate 102, and an isolation structure 106 over thesubstrate 102 and surrounding the first S/D region 104 a. The device 500further includes two rod-shaped mesas over the first S/D region 104 aand extending upwardly along the “z” direction. The middle portions ofthe two rod-shaped mesas provide two transistor channels 104 b. The topportions of the two rod-shaped mesas provide two S/D regions 104 c. Thefirst S/D region 104 a, the channel 104 b, and the second S/D region 104c are arranged vertically over the substrate. A gate 108 wraps aroundthe transistor channels 104 b. Therefore, the device 500 is a verticalgate-all-round (VGAA) device. The device 500 further includes the ILDlayer 110 over the substrate 102 and the isolation structure 106,filling in the spaces between the various structures. In embodiments,the ILD layer 110 may include one or more dielectric layers. Thematerial and composition of the various elements 102, 104 a-c, 106, 108,and 110 are similar to those of the device 100. Exemplary processes offorming the device 500 prior to the S/D contact formation can be foundin U.S. Pat. No. 8,742,492 entitled “Device with a Vertical GateStructure” and U.S. Pat. No. 8,754,470 entitled “Vertical TunnelingField-Effect Transistor Cell and Fabricating the Same,” the contents ofwhich are hereby incorporated by reference in their entirety.

Another exemplary vertical multi-gate device, the device 600, is shownin FIGS. 14A and 14B. FIG. 14A is a schematic perspective view of thedevice 600 and FIG. 14B is a top view of the device 600 (with the ILDlayer 110 removed). Many respects of the device 600 are similar to thoseof the device 500. One difference between the two devices lies in theshape of the mesa over the first S/D region 104 a. The device 600 has abar-shaped vertical mesa where the channel 104 b and the second S/Dregion 104 c are included or formed therein. The device 600 is also aVGAA device. The devices 500 and 600 may be considered two variants ofthe same general type of devices, and will be discussed collectivelybelow. In particular, FIGS. 15-18 show cross-sectional views of thedevices 500/600 along the “C-C” line of FIG. 13A for the device 500 andalong the “D-D” line of FIG. 14A for the device 600. FIG. 15 illustratesthe devices 500/600 prior to the S/D contact formation.

At operation 54, the method 50 (FIG. 12) etches the ILD layer 110 andthe isolation structure 106 to form an opening 112. Referring to FIG.16, the opening 112 exposes a portion of the top surface 104 a′ and aportion of the sidewall 104 a″ of the first S/D region 104 a. Since theisolation structure 106 initially surrounds the first S/D region 104 a,it is partially removed in the etching process to expose the surface 104a″. The opening 112 is deeper than conventional S/D contact holes whichtypically stop at the top surface 104 a′. Therefore, the opening 112offers more contact areas to the first S/D region 104 a than theconventional S/D contact holes. In some embodiments, the opening 112 mayexpose more than two surfaces of the first S/D region 104, for example,a top surface and two sidewall surfaces, to further increase the contactarea. The etching processes may include a suitable wet etch, dry(plasma) etch, and/or other processes. In embodiments, the devices500/600 include a contact etch stop (CES) layer over the first S/Dregion 104 a and underneath the ILD layer 110. To further thisembodiment, the partial removal of the CES layer can be performed in amanner similar to that of the method 10 with reference to FIG. 3B.

At operation 56, the method 50 (FIG. 12) forms a first contact layer 114in the opening 112. Referring to FIG. 17, the first contact layer 114 isformed over the surfaces of the opening 112. In particular, it is formedover the two surfaces 104 a′ and 104 a″ (FIG. 16) of the first S/Dregion 104 a. The first contact layer 114 has a conformal profile. In anembodiment, the first contact layer 114 has a thickness ranging fromabout 2 nm to about 10 nm. The material and formation of the firstcontact layer 114 are similar to those discussed with reference to FIGS.4A and 4B. In various embodiments, the material of the first contactlayer 114 offers low or negligible energy barrier for charge carriersflowing into and out of the transistor channel. The first contactmaterial coupled with increased contact areas to the first S/D region104 a reduces the S/D contact resistance thereof.

At operation 58, the method 50 (FIG. 12) forms a second contact layer116 in the opening 112 over the first contact layer 114. Referring toFIG. 18, an S/D contact 118 s is formed in the opening 112, conductivelyconnecting to the first S/D region 104 a. The S/D contact 118 s includesthe first contact layer 114 and the second contact layer 116. Otherrespects of the contact 118 s are similar to those discussed withreference to FIGS. 5A and 5B. In the present embodiment, the S/D region104 a is a source region of the device 500/600 and the S/D contact 118 sis a source contact.

At operation 60, the method 50 (FIG. 12) performs further steps tocomplete the fabrication of the devices 500/600. For example, operation50 may form another S/D contact 118 d electrically connecting the secondS/D region 104 c, as shown in FIG. 19. Referring to FIG. 19, the S/Dcontact 118 d also includes a first contact layer 114 and a secondcontact layer 116, wherein the first contact layer 114 wraps aroundthree sides of the S/D region 104 c. In an embodiment, the S/D contact118 d is formed by etching the ILD layer 110 to form an opening thatexposes the three sides of the S/D region 104 c (similar to theoperation 54), forming the first contact layer 114 in the opening (theoperation 56), and forming the second contact layer 116 over the firstcontact layer 114 (the operation 58). In an embodiment, the S/D contacts118 s and 118 d are formed by the same process that includes the etchingof the ILD layer 110 (the operation 56), the forming of the firstcontact layer 114 (the operation 56), and the forming of the secondcontact layer 116 (the operation 58). In the present embodiment, the S/Dregion 104 c is a drain region of the device 500/600 and the S/D contact118 d is a drain contact.

The method 50 (FIG. 12) may perform further steps to complete thefabrication of the devices 500/600. For example, it may form a gatecontact electrically connecting the gate 108, and form metalinterconnects connecting the multi-gate FET to other portions of thedevice 100 to form a complete IC.

FIG. 20 shows an embodiment of the device 700, constructed according tovarious aspects of the present disclosure. Some differences between thedevice 700 and the device 500/600 (FIG. 18) are noted. One difference isthat the first contact layer 114 is formed over the entire top surfaceof the source region 104 a not covered by the vertical mesa. Anotherdifference is that the source contact 118 s in the device 700 optionallyincludes a barrier metal layer 116 a between the second contact layer116 and the layers surrounding the second contact layer 116. Notably,the source contact 118 s contacts at least a portion of the top surfaceand a portion of the sidewall surface of the source region 104 a,reducing the source contact resistance. The barrier metal layer 116 a isconductive and has a conformal profile, similar to the first contactlayer 114 of FIG. 18. In an embodiment, the barrier metal layer 116 aincludes a metal nitride (e.g., TaN).

The contact layers 116 a and 116 of the device 700 may be formed with anembodiment of the method 50 (FIG. 12), as discussed above, wherein thebarrier metal layer 116 a is deposited (e.g., by a CVD or PVD process)in the opening 112 (FIG. 16) followed by the deposition of the secondcontact layer 116 (FIG. 18). The formation of the first contact layer114 is briefly discussed below. In an embodiment, first, a hard mask 120and a vertical mesa (104 b-c) are formed over the source region 104 a(FIG. 21) using various deposition and etching processes. Next, a spacerfeature 122 is formed around the vertical mesa (FIG. 22). The spacerfeature 122 may be formed by CVD of silicon nitride followed by reactiveion etching, in one example. Next, the first contact layer 114 is formedover the source region 104 a (FIG. 23) using one of the techniquesdiscussed above. Next, the spacer feature 122 is removed (FIG. 24), andthe ILD layer 110 and the gate 108 are formed (FIG. 25). Subsequently,an embodiment of the method 50 (FIG. 12) is used to form the barriermetal layer 116 a and the second contact layer 116 as shown in FIG. 20,and may further form a drain contact 118 d as shown in FIG. 19.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device andthe formation thereof. For example, source/drain (S/D) contact holesetched according to embodiments of the present disclosure provide largercontact areas to S/D regions of a transistor than conventional S/Dcontact holes. Contact holes of the present disclosure expose multiplesurfaces of the S/D regions, such as a top surface, one or more sidewallsurfaces, and/or surfaces all-around. The larger contact areascontribute to lower S/D contact resistance. Furthermore, S/D contactsformed according to embodiments of the present disclosure include twolayers of contact materials. In particular, the first contact layer isconformal and is in direct contact with the semiconductor material ofthe respective S/D region. The material(s) of the first contact layeroffers low or negligible energy barrier for charge carriers to flow intoor out of the transistor channel. The contact material coupled with thelarge contact area provides ultra-low contact resistivity. Inexperiments, contact resistivity in the range of 1×e⁻¹⁰ to 1×e⁻⁸ ohm·cm²has been achieved.

In one exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes a substrate,first and second source/drain (S/D) regions, a channel between the firstand second S/D regions, a gate engaging the channel, and a contactfeature connecting to the first S/D region. The contact feature includesa first contact layer and a second contact layer over the first contactlayer. The first contact layer has a conformal cross-sectional profileand either is in contact with the first S/D region on at least two sidesof the first S/D region or wraps around the first S/D region.

In another exemplary aspect, the present disclosure is directed to amethod of forming a contact in a vertical gate-all-around (VGAA) device.The method includes receiving a VGAA device having a substrate, a firstsource/drain (S/D) region over the substrate, an isolation structureover the substrate and surrounding the first S/D region, a channel overthe first S/D region, a second S/D region over the channel, a gatewrapping around the channel, and a dielectric layer over the isolationstructure and the first S/D region. The method further includes etchingthe dielectric layer and the isolation structure to form an opening,wherein the opening exposes at least two sides of the first S/D region.The method further includes forming a first contact layer in theopening, wherein the first contact layer has a conformal cross-sectionalprofile and is in contact with the first S/D region. The method furtherincludes forming a second contact layer in the opening over the firstcontact layer.

In another exemplary aspect, the present disclosure is directed to amethod of forming a contact in a multi-gate semiconductor device. Themethod includes receiving a multi-gate semiconductor device having asubstrate, first and second source/drain (S/D) regions, a channelbetween the first and second S/D regions, a gate engaging the channel,and a dielectric layer over the first S/D region. The method furtherincludes etching the dielectric layer to form an opening, wherein theopening exposes at least two sides of the first S/D region or wrapsaround the first S/D region. The method further includes forming a firstcontact layer in the opening, wherein the first contact layer has aconformal cross-sectional profile and is in contact with the first S/Dregion. The method further includes forming a second contact layer inthe opening over the first contact layer.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A method of forming a contact in a verticalgate-all-around (VGAA) device, comprising: receiving a VGAA device, theVGAA device having: a substrate; a first source/drain (S/D) region overthe substrate; an isolation structure over the substrate and surroundingthe first S/D region; a channel over the first S/D region; a second S/Dregion over the channel; a gate wrapping around the channel; and adielectric layer over the isolation structure and the first S/D region;etching the dielectric layer and the isolation structure to form anopening, wherein the opening exposes at least two sides of the first S/Dregion; forming a first contact layer in the opening, wherein the firstcontact layer has a conformal cross-sectional profile and is in contactwith the first S/D region; and forming a second contact layer in theopening over the first contact layer.
 2. The method of claim 1, whereinthe first contact layer includes one of InAs, InGaAs, InP, and Ge. 3.The method of claim 1, wherein the first contact layer includes asemiconductor-metal alloy.
 4. The method of claim 1, wherein the openingexposes a top surface and two sidewall surfaces of the first S/D region.5. A method of forming a contact in a multi-gate semiconductor device,comprising: receiving a multi-gate semiconductor device having: asubstrate; first and second source/drain (S/D) regions; a channelbetween the first and second S/D regions; a gate engaging the channel;and a dielectric layer over the first S/D region; etching the dielectriclayer to form an opening, wherein the opening exposes at least two sidesof the first S/D region; forming a first contact layer in the opening,wherein the first contact layer has a conformal cross-sectional profileand is in contact with the first S/D region; and forming a secondcontact layer in the opening over the first contact layer.
 6. The methodof claim 5, wherein the first contact layer includes one of III-Vsemiconductors.
 7. The method of claim 5, wherein: the first contactlayer includes a material selected from the group consisting oftitanium, cobalt, nickel, nickel cobalt, and germanium.
 8. The method ofclaim 5, wherein the multi-gate semiconductor device further includes acontact etch stop (CES) layer between the dielectric layer and the firstS/D region, wherein the etching of the dielectric layer includes:etching the CES layer within the opening to expose the at least twosides of the first S/D region.
 9. The method of claim 5, wherein thefirst S/D region is a fin structure having opposing sidewall surfaces.10. The method of claim 9, wherein forming the first contact layer inthe opening includes forming the first contact layer directly on theopposing sidewall surfaces such that the first contact layer physicallycontacts the opposing sidewall surfaces.
 11. The method of claim 5,further comprising: epitaxially growing a semiconductor material on thefirst S/D region to form a diamond shaped S/D feature, the diamondshaped S/D feature having a first side, a second side, a third side, anda fourth side, and wherein forming the first contact layer in theopening includes forming the first contact layer directly on the firstside, the second side, the third side, and the fourth side.
 12. Themethod of claim 5, further comprising: forming a semiconductor materialon the first S/D region to form a multi-sided S/D feature, themulti-sided S/D feature having a first side surface and a second sidesurface, wherein forming the first contact layer in the opening includesforming the first contact layer directly on the first side surfacewithout forming the first contact layer directly on the second sidesurface.
 13. A method comprising: forming a gate stack over a substrate;forming a source/drain feature over the substrate, the source/drainfeature having a first side surface, a second side surface, and a thirdside surface; forming a dielectric layer over the source/drain featureand the gate stack; forming a trench through the dielectric layer toexpose at least the first side surface, the second side surface, andthird side surface; forming a first contact layer in the trench suchthat the first contact layer physically contacts the first side surface,the second side surface, and third side surface; and forming a secondcontact layer in the trench over the first contact layer.
 14. The methodof claim 13, wherein after forming the first contact layer in thetrench, at least one of the first side surface, the second side surface,and the third side surface includes a portion that physically contactsthe dielectric layer.
 15. The method of claim 13, wherein the first sidesurface intersects the second side surface, and wherein the third sidesurface intersects the second side surface without intersecting thefirst side surface.
 16. The method of claim 13, further comprisingforming a fin structure over the substrate, and wherein forming thesource/drain feature over the substrate includes forming thesource/drain feature directly on the fin structure.
 17. The method ofclaim 16, wherein the first side surface and the second side surfacedirectly interface with the fin structure.
 18. The method of claim 13,wherein the first contact layer includes a III-V semiconductor materiallayer, and wherein the second contact layer includes a metallic materiallayer.
 19. The method of claim 13, further comprising forming anothersource/drain feature over the substrate, and wherein forming the trenchthrough the dielectric layer further exposes the another source/drainfeature, and wherein forming the first contact layer in the trenchfurther includes forming the first contact layer directly on the anothersource/drain feature such that the first contact layer physicallycontacts the another source/drain feature, the first contact layerextending continuously from the source/drain feature to the anothersource/drain feature.
 20. The method of claim 19, wherein thesource/drain feature physically contacts the another source/drainfeature.